Thermoplastic elastomers (TPEs) are polymeric materials which possess both plastic and rubbery properties. They have elastomeric mechanical properties but, unlike conventional thermoset rubbers, they can be re-processed at elevated temperatures. This re-processability is a major advantage of TPEs over chemically crosslinked rubbers since it allows recycling of fabricated parts and results in a considerable reduction of scrap.
In general, two main types of thermoplastic elastomers are known. Block copolymer thermoplastic elastomers contain "hard" plastic segments which have a melting point or glass transition temperature above ambient as well as "soft" polymeric segments which have a glass transition considerably below room temperature. In these systems, the hard segments aggregate to form distinct microphases and act as physical crosslinks for the soft phase, thereby imparting a rubbery character at room temperature. At elevated temperatures, the hard segments melt or soften and allow the copolymer to flow and to be processed like an ordinary thermoplastic resin.
Alternatively, a thermoplastic elastomer referred to as a simple blend (physical blend) can be obtained by uniformly mixing an elastomeric component with a thermoplastic resin. When the elastomeric component is also cross-linked, a thermoplastic elastomer known in the art as a thermoplastic vulcanizate (TPV) results. Since the crosslinked elastomeric phase of a TPV is insoluble and non-flowable at elevated temperature, TPVs generally exhibit improved oil and solvent resistance as well as reduced compression set relative to the simple blends.
Typically, a TPV is formed by a process known as dynamic vulcanization, wherein the elastomer and the thermoplastic matrix are mixed and the elastomer is cured with the aid of a crosslinking agent and/or catalyst during the mixing process. A number of such TPVs are known in the art, including some wherein the crosslinked elastomeric component can be a silicone polymer while the thermoplastic component is an organic, non-silicone polymer (i.e., a thermoplastic silicone vulcanizate or TPSiV). In such a material, the elastomeric component can be cured by various mechanisms, but it has been shown that the use of a non-specific catalyst, such as an organic peroxide, can also result in at least a partial cure of the thermoplastic resin itself, thereby reducing or completely destroying ability to re-process the composition (i.e., it no longer is a thermoplastic elastomer). In other cases, the peroxide can lead to the partial degradation of the thermoplastic resin. To address these problems, elastomer-specific crosslinkers, such as organohydrido silicon compounds, can be used to cure alkenyl-functional elastomers.
Arkles, in U.S. Pat. No. 4,500,688, discloses semi-interpenetrating networks (IPN) wherein a vinyl-containing silicone fluid having a viscosity of 500 to 100,000 cS is dispersed in a conventional thermoplastic resin. Arkles only illustrates these semi-IPNs at relatively low levels of silicone. The vinyl-containing silicone is vulcanized in the thermoplastic during melt mixing according to a chain extension or crosslinking mechanism which employs a silicon hydride-containing silicone component. This dsclosure is expanded by Arkles in U.S. Pat. No. 4,714,739 to include the use of hybrid silicones which contain unsaturated groups and are prepared by reacting a hydride-containing silicone with an organic polymer having unsaturated functionality. In U.S. Pat. No. 4,970,263, Arkles et al. further extend the above concepts to systems wherein the semi-IPN is crosslinked by hydrolysis of alkoxysilyl groups on the polymer network. The product is said to have improved chemical resistance and mechanical properties as well as excellent temperature resistance and electrical properties.
In a copending application to Gornowicz et al. entitled "Thermoplastic Silicone Elastomers" (Ser. No. 09/034,089), we also teach the preparation of TPSiVs wherein silicone gum is dispersed in an organic resin and subsequently dynamically vulcanized therein via a hydrosilation cure system. Under certain conditions, such systems were shown to have significantly improved mechanical properties over the corresponding simple blends of resin and silicone gum in which the gum was not cured. Unfortunately, only polyolefin or poly(butylene teraphthalate) resins were suitable and attempts to prepare a similar TPSiV based on other resins proved unsuccessful, mechanical properties of the dynamically vulcanized system being comparable to those of the corresponding simple blend. Moreover, many resins may contain groups that can "poison" (i.e., inactivate) the platinum catalyst used to promote the hydrosilation reaction. Moreover, some resins contain residual unsaturation (e.g., styrene-butadiene-styrene, styrene-isoprene-styrene) which would react with the SiH-functional cure agent in the above mentioned systems, thereby depleting this crosslinker and potentially crosslinking the resin itself.
Thus, although the above cited publications disclose the preparation of thermoplastic elastomer compositions using various thermoplastic resins as the matrix and a dispersed silicone phase which is dynamically vulcanized therein, neither these publications, nor any art known to applicants, teach TPSiVs wherein the silicone is cured by a condensation reaction which is not sensitive to the above mentioned poisoning phenomenon.